I've only done it once and found it doesn't steep well cold. If I'm feeling like a cup I'll make extra and just let it sit and cool, stick it in the fridge (bag in) and transfer to a bigger container in the morning. It's stronger that way.

I've done it a few times for two parties thus far, steeping each batch in the fridge three to four hours before serving/straining. It keeps well when constantly cooled but it does far, far better when steeped similiar to sun teas (mason jar + tea + outside in the sun all day = beauty) as room temperature water become saturated with tea/minerals/etc quicker than chilled as per basic chemistry. Of course, cold steeping in a sealed container gets fewer of the heat-activated ingredients ingrained in other teas though I'm uncertain as to any differences with rooibos as it tastes and looks the same no matter my steeping method. That's a bit of research that I've never searched for nor considered, let alone thinking that a study has been conducted on it. Ah well, there's my experiences.